1. Field of the Invention
This invention relates to rechargeable hydride batteries. More particularly, the invention discloses a method to make improved high capacity rechargeable batteries comprising a metal oxide-hydride couple in which the negative electrode is made of an improved hydride/hydrogen storage electrode.
2. The Related Art
Sapru et. al. in U.S. Pat. No. 4,551,400 and Rechman et al. in U.S. Pat. No. 4,716,088 reported a rechargeable hydride battery using pseudo TiV.sub.2 -type hydrogen storage alloy as the active material of the negative electrode. Their alloys, Ti--V--Ni, Ti--Zr--V--Ni and Ti--Cr--V--Ni have short cycle life, high self-discharge rate, and/or are very difficult to activate. U.S. Pat. Nos. 4,728,586, 5,096,667, 5,104,617, and 5,238,756 disclose a Ti--Zr--V--Ni--Cr-- based alloy for hydride electrode. This kind of alloy still has some weaknesses. The alloy disclosed in these patents contains Ti+Zr from 16.5 at. % to 37.9 at. % and V+Ni from 34.8 to 70. 98 at. %. As a result, as given in their examples, the alloys disclosed contain a substantial amount of vanadium metal which is very expensive and still has a high corrosion rate in an alkaline medium. Consequently, the cell made by a conventional method still has high internal pressure during overcharge and cycle life is shortened. Treatment with alkaline etching will not be able to solve the intrinsic fundamental weakness of these alloys. Moreover, the corrosion leads to the formation of an oxide layer and low surface area. This does not result in reduced self discharge. In contrast, it leads to poor charging efficiency, poor rate capability and higher internal pressure in a sealed cell. These patents do not disclose a method to prepare a hydride cell using the alloy disclosed. More importantly, these prior arts do not teach how to fabricate a workable rechargeable cell using the alloy thereof. Consequently their "sample" cells have poor capacity and poor cycle life as shown in these patents.
U.S. Pat. Nos. 4,849,205, 4,946,646 and 5,006,328 disclosed hydride storage electrode alloys and suggested the use of the alloy to be used in an electrochemical cell. However, little or no information was given regarding how to fabricate a workable rechargeable electrochemical cell using these alloys.
More recently, U.S. Pat. No. 5,242,656 issued on Sep. 7, 1993, to Zhang et al, Zhang discloses a hydride battery using a CaNi.sub.5 -type alloy, MmNi.sub.5-x-y-z-u A.sub.x B.sub.y C.sub.z D.sub.u, where Mm is mischmetal, A=Mn, Sn, or V; B=Cr, Co, Ti, Zr, or Si; C=Al, Mg, or Ca; D=Li, Na, or K; and 0&lt;x&lt;0.95, 0&lt;y&lt;1, 0&lt;z&lt;0.7, 0.1&lt;u&lt;0.9; or a ternary ally, Ti.sub.2 Ni.sub.1-u D.sub.u, where D=Li, Na, or K; and 0.04&lt;u&lt;0.9. Zhang claimed their materials have a better result. However, the use of mischmetal which is a mixture of rare earth metals and the CaNi.sub.5 structure, limits the availability of the raw material source and alloy selection. Furthermore, rare earth metals are very sensitive to oxygen and moisture, and the alloys have high corrosion rates. Therefore, the preparation process requires a lot of precautions and a handful of procedures. Moreover, the capacity of the alloy is not high. On the other hand, the use of Ti.sub.2 Ni with a modifier of Li, Na or K to partially replace Ni has even more weakness. It is not easy to introduce Li, Na or K into Ti.sub.2 Ni body, thus the alloy is still not easy to activate and the cycle life will drop drastically after these alkali metals dissolve in alkaline solution. Furthermore, the introduction of alkali metals, especially Li into Ti.sub.2 Ni, only results in a more stable hydride. The rate capability and usable capacity are very poor.
To solve these problems, the present invention discloses a method to make a newly improved hydride battery using an improved hydrogen storage/hydride electrode.